Sad to say but it boils down to the other providers. The big problem with modern healthcare is that too much litigation happens these days.
Endos and SRS surgeons are covering themselves. Example scenario:
A person who seems to be MTF presents to an endo, dressed well, asks for HRT, endo thinks this person is genuine GID, prescribes the hormones.
This person then goes for SRS, then regrets the whole deal, and then sues them. Armchair Dr (lawyer) points out that person hasn't been checked/diagnosed, wins lawsuit and profits.
Now, what I said doesn't happen that often. But it does, and as a result, the expected duty of care from a health provider has risen and referral rates, and orders for diagnostics have sky-rocketed as a result. So unfortunately, even if the endo or SRS surgeon is sure of your decision, they're basically required to make you get a recommend letter from a medical indemnity insurance point of view.
As far as when actual _helpful_ therapy becomes unnecessary?
My feeling on it is when you're comfortable in your own skin. I had my last therapy session over 9 years ago, and only started HRT this year. My GP and endo were happy to start me on low dose as I was quite specific about what I was aiming to achieve. Further therapy for me will basically only retread the same topics over and is likely to be unproductive at this point in time.